The present invention relates in general to apparatuses and methods for separating fractions of a particulate material. More particularly, the present invention relates to apparatuses and methods for utilizing air to separate components of a particulate material on the basis of differing attributes.
The separation of a particulate material into various fractions on the basis of density is performed in many industrial processes. In the mining industry, heavy minerals are concentrated from ores for extraction. In agriculture, grain is separated from chaff and leaves are separated from stalks by a current of air that lifts the lighter chaff or leaves away from the grain or stalks. In the wood pulping industry, a device known as an air density separator has been employed to separate light wood chips from chips containing knots which are more dense.
An air density separator uses a vertical separation chamber through which a stream of air is drawn. Wood chips to be separated are metered by an auger into the separation chamber where the high velocity air stream disperses the chips evenly over the chamber. The more dense knots fall through the uprising current of air and are rejected. The lighter chips are drawn from the separation chamber by the flow of air and separated from the air by a cyclone.
In the production of paper from wood fibers, the wood fibers must be freed from the raw wood. One widely used method of accomplishing this is to process the wood fibers in a cooking liquor so that the material holding the fibers together, lignin, is dissolved. To achieve rapid and uniform digestion by the cooking liquor, the wood, after it has been debarked, is passed through a chipper that reduces the raw wood to chips.
As a natural consequence of the harvesting and processing of pulp logs, some sand, rocks, and tramp metal find their way into the raw wood chips. Further, a certain percentage of the raw wood is comprised of knots which are in general undesired in the papermaking process because they add dark fibers that increase the bleaching requirement and because they contain resinous material. The knots, which are typically of a higher density because the wood is dense and resinous, together with tramp metal and rocks, must be separated from the raw wood chips before further processing.
One highly successful method of accomplishing this separation is the air density separator. In one known successful system, chips are supplied by a metering screw conveyor infeed to a separation chamber through which a stream of air is drawn. The chips are entrained in the air stream while the higher density knots, stones and tramp metal move against the current of air under the force of gravity. The acceptable chips and air then pass into a cyclone where the chips are separated from the air, the air being drawn by a vacuum into a fan and exhausted.
While the air density separator is the most effective and discriminating system available, it has some less desirable features. First, it requires a baghouse to remove dust from the exhaust air. The baghouse is expensive and requires labor intensive maintenance. Further, use of a baghouse results in higher energy cost because of the air pressure necessary to move the air through the filters. Conventional air density separators using air velocities of 4,000 to 5,000 feet per minute function well at dispersing and separating larger wood chips from knots, rocks, and tramp metal. However, separation of small chips from sand and dust requires a lower velocity air flow. Here the conventional method of dispersing the material to be separated in the air stream is not effective.
What is needed is an air density separator that eliminates the requirement for a baghouse and can process lightweight materials in a low velocity air stream.